Ultrastructural Changes of the Egg Apparatus Associated with Fertilization and Proembryo Development of Soybean, Glycine max (Fabaceae)

As part of a study involving pod retention in soybean, Glycinemax (L.) Merr., we investigated changes occurring in the eggapparatus of non-abscised flowers from the time immediatelypreceding fertilization through early embryogeny. Prior to theentry of the pollen tube into the embryo sac, one of the synergidsbegins to degenerate as evidenced by increased electron densityand a loss of volume. This cell serves as the site of entryfor the pollen tube. The cytoplasm of the second, or persistentsynergid, remains unaltered until after fertilization. Bothsynergids contain, in addition to a filiform apparatus, a singleunidentified inclusion of flocculent material located in thechalazal portion of each cell. The zygote can be distinguishedfrom the egg by its consistently narrow wall; and it dividesto form a proembryo, a mass of cells not yet differentiatedinto embryo proper and suspensor. The basal cells of the proembryoare more vacuolate than the apical ones, characteristic of thebasal vacuolation of both egg and zygote. Cells of the proembryoare connected to one another via plasmodesmata, and with theexception of the basal-most cell, are isolated symplasticallyfrom the surrounding endosperm. Wall ingrowths frequently occurin certain cells of the proembryo, notably those cells in contactwith the degenerate synergid and embryo sac wall. At a laterstage of ontogeny, by which time the globular embryo properhas become distinct from the suspensor, the wall ingrowths areconcentrated in the suspensor. Glycine max, soybean, embryogeny, synergids